1. Field of the Invention
This invention relates to telecommunications systems, and more particularly to a method and apparatus for storing and retrieving performance data collected by a network interface unit in a telecommunications system.
2. Description of Related Art
Public Switched Telephone Networks (PSTN) commonly utilize Time Division Multiplexing (TDM) transmission systems to communicate both voice and data signals over a digital communications link. For example DS1 (digital signal level 1) data paths are currently used to carry both voice and data signals over a single transmission facility. DS1 paths carry DS1 signals which are transmitted at a nominal rate of 1.544 Mb/s (e.g., 8,000 frames of 193 bits per second, representing 24 channels of 8 bits plus one overhead bit per frame). DS1 paths reduce the number of lines required to carry voice and data signals by multiplexing the 24 channels on a signal data path. Data paths, such as DS1 paths, have a portion of the data transmission capability assigned to communicating customer information from one end of the data path to the other. This portion of the transmission capability is commonly referred to as the "payload" (i.e., 192 bits of each of the 8000 frames). In addition, another portion of the transmission capability is assigned to overhead functions, such as error detection and maintaining the data path. This portion of the transmission capability is commonly referred to as "overhead" (i.e., the 193rd bit of each of the 8000 frames).
New equipment typically provides users with an option for selecting between one of two commonly used formats; super frame (SF) format and extended super frame (ESF) format. However, most of the equipment currently in use provides only the SF frame format. Network interface units (NIUs) provided by Applied Digital Access have the ability to change the superframe (SF) formatted signal (having only 12 frames) to an extended superframe signal (ESF) having 24 frames.
The principle advantage provided by the use of ESF formatted signals is that a portion of the 8 kb/s (kilobit per second) overhead bits that is dedicated to framing in SF signals are made available for other purposes. That is, in the ESF frame format, the 8 kb/s overhead capacity is divided into three independent channels having capacities as indicated below:
fps (framing): 2 kb/s PA1 CRC (error checking): 2 kb/s PA1 DL (data link): 4 kb/s PA1 HDLC flags (idle code): 8 bits in length PA1 unscheduled message: 16 bits in length PA1 scheduled message: 104 bits in length
The overhead capacity of an ESF formatted DS1 signal therefore is occupied by three separate and distinct signals. A 6 bit fps pattern is repeated once per extended superframe. An extended superframe is equal in length to 24 super frames (i.e., 193 bits.times.24=4,632). Therefore, one fps bit of the 6 bit pattern occurs each 772 bits. A 6 bit CRC is also repeated once per extended superframe. Three types of patterns carried by the DL are as follows:
The DS1 path 30 includes hardware used by the network providers to transmit DS1 digital signals between equipment controlled by different data users. The DS1 path 30 as shown in FIG. 1 is implemented by a T1 line. However, other facilities, such as coaxial cables, fiber optic cables, and microwave links may be used by providing an appropriate transport interface between the Channel Service Unit (CSU) 20 and the facility.
DS1 signals may be transmitted over a dedicated point-to-point network as simple as the one shown in FIG. 1 utilizing twisted wire pairs and repeaters spaced at intermediate points. Alternatively, the network may be as complex as the one shown in FIG. 2 which utilizes a combination of twisted wire pairs and repeaters, multiplexers, Digital Cross-connect Systems (DCS), Add Drop Multiplexers (ADM), Fiber Optic Terminals (FOT), Coaxial Cable, Microwave, Satellite, or any other transmission media capable of transporting a DS1 signal. In some instances, DS1 signals may be carried over a network similar to the point-to-point network, but having the added capability to switch the DS1 signal (in a DCS or ADM) in a manner similar to the PSTN.
While DS1 transmission systems such as the system shown in FIG. 1 are well-known, customers more typically communicate using a public DS1 network, as shown in FIG. 2. In the DS1 transmission system shown in FIG. based on the location of the equipment. Essentially, the equipment is broken into three categories: (1) the Customer Premises Equipment (CPE) 40; (2) the Local Exchange Carrier (LEC) equipment, which comprises the local loop 42 and the central office equipment; and (3) the InterExchange Carrier (IEC) 52. CPE 40 belongs to the network user (or customer). The customer that owns the CPE 40 is responsible for both its operation and maintenance. The customer must ensure that its equipment provides a healthy and standard DS1 digital signal to the local exchange carrier equipment 42. The equipment 40 on the customer premises typically consists of DS1 multiplexers, digital Private Branch Exchanges (PBXs), and any other DS1 terminating equipment which connects to a CSU 20 at the CPE site. The local exchange carrier equipment 42 connects the CPE 40 with the central office 44 and the IEC 52. LECs assume responsibility for maintaining equipment at the line of demarcation between the CPE 40 and the local loop 42.
As shown in FIG. 2, an NIU 50 may be coupled between the CPE 40 and the LEC equipment 42. The NIU 50 represents the point of demarcation between the CPE 40 and the LEC equipment 42 (which comprises local loop equipment 42, and Central Office (CO) equipment 44). Prior art NIUs may be relatively simple devices which allow network technicians to minimally test the operation and performance of both the CPE 40 and the DS1 network 52 or they may be more sophisticated devices.
Independent of whether the DS1 transmission system is simple (FIG. 1), complex (FIG. 2), or switched, all the circuits and network equipment required to transmit a DS1 signal must be tested and maintained to operate at maximum efficiency. Currently, when a problem is reported on a DS1 path, one method for sectionalizing the path requires sending a technician to several points along the path to collect data. The collected data is then reviewed by the technician in an attempt to determine the point of origin of a problem. Since the equipment that makes up the path is physically distributed over a large geographic region (equipment at one end of the path may be thousands of miles from equipment at the other end of the path) it is typical for several technicians to become involved in the sectionalization of the path. Each technician must be highly skilled and trained in order to collect the data from the various points along the path.
In many cases the responsibility for sectionalizing the DS 1 path must be shared by the LEC, IEC, and customer, since each party is responsible for maintaining a portion of the equipment that makes up the path. Accordingly, each party incurs an expense when equipment maintained by any of the other parties experiences a problem.
In some cases, the revenue bearing signals being transmitted over the path must be disrupted in order to perform tests which generate data to be collected by a technician. Such "out-of-service testing" causes "live" traffic to be removed from the DS1 link before testing commences. In out-of-service testing, a test instrument transmits a specific data pattern to a receiving test instrument that anticipates the sequence of the pattern being sent. Any deviation from the anticipated pattern is counted as an error by the receiving test instrument. Out-of-service testing can be conducted on a "point-to-point" basis (i.e., by sending a test pattern from a first location to a second location) or by creating a "loop-back" (i.e., sending a test pattern from a first location, and requesting that a device along the signal path loop that test pattern back over the return path to the first location). Point-to-point testing requires two test instruments (a first instrument at one end of the DS1 transmission system, and a second instrument at the other end of the DS1 transmission system). By simultaneously generating a test data pattern and analyzing the received data for errors, the test instruments can analyze the performance of a DS1 link in both directions.
Loop-back testing is often used as a "quick check" of circuit perfonnance or when attempting to isolate faulty equipment. In loop-back testing, a single test instrument sends a "loop-up" code to a loop back device, such as the CSU at the far end, before data is actually transmitted. The loop-up code causes all transmitted data to be looped back by the CSU in the direction toward the test instrument. By analyzing the received data for errors, the test instrument measures the performance of the link up to and including the far end CSU. Because loop-back testing only requires a single test instrument, and thus, only one operator, it is a convenient testing means.
Both point-to-point and loop-back tests allow detailed measurements of any DS1 transmission system. However, because both testing methods require that live, revenue-generating traffic be interrupted, they are undesirable. Thus, out-of-service testing is inherently expensive and undesirable. It is therefore desirable to perform in-service monitoring of "live" data to measure the performance and viability of DS1 transmission systems. Because in-service monitoring does not disrupt the transmission of live, revenue-generating traffic, it is suitable for routine maintenance and it is preferred by both the LECs and their customers.
Equipment 44 within the central office may be capable of monitoring for various DS1 signal requirements to reduce the amount of effort required by technicians. In addition, equipment within the DS1 path provides maintenance and alarm signals indicative of particular conditions on the incoming and outgoing signals. These signals are defined by standards established by the American National Standards Institute for operation of DS1 communications links (ANSI T1.403-1995 and ANSI T1.408, et. al, each of which are herein incorporated by reference). These signals include (1) RAI, which indicates that the signal that was received by the CPE equipment from the NIU 50 was lost (the detailed requirements for sending RAI are contained in ANSI T1.231, which is herein incorporated by reference); and (2) AIS, which is an unframed, all-ones signal which is transmitted to the network interface upon loss, or in response to the presence of a signal defect of the originating signal (the signal received from the network), or when any action is taken that would cause a service disruption (such as loopback). In addition, detection of signal defects in the digital hierarchy above DS1 causes an AIS signal to be generated. The AIS signal is removed when the condition that triggered the AIS is terminated.
In addition to these indications ANSI T1.403 defines a performance report message (PRM) which is sent each second using a format which is detailed in the ANSI standard. PRMs contain performance information within a "message field" portion of the PRM. The performance information indicates the performance of the circuit in each of four previous one-second intervals. For example, counts of cyclic redundancy checking (CRC) errors are accumulated in each contiguous one-second interval by the reporting equipment. In accordance with the ANSI standard, one bit (designated "G1" by the ANSI standard) within the variable portion of the PRM indicates that one CRC error had occurred within the last second of transmission. At the end of each one-second interval, a modulo-4 counter within the variable portion of the PRM is incremented, and the appropriate performance bits are set within the remainder of the variable portion of the report in accordance with the PRM format provided in the specification. Other bits indicate that: (1) between 2 and 5 CRC events had occurred; (2) between 6 and 10 CRC events had occurred; (3) between 11 and 100 CRC events had occurred; (4) between 101 and 319 CRC events had occurred; (5) greater than 319 CRC events had occurred; (6) at least one severely errored framing event had occurred; (7) at least one frame synchronization bit error event had occurred; (8) at least one line code violation event had occurred; and (9) at least one slip event had occurred. The number and type of Events indicates the quality of transmission. Each Event is defined within ANSI T1.403. Performance reports may be also be generated and transmitted in accordance with AT&T PUB 54016 Performance Reporting, which is herein incorporated by reference.
The use of PRMs reduces the effort required of technicians, since performance information is collected and reported without the need to dispatch a technician. In addition, the information is collected and reported non-intrusively in real-time. PRMs provide information that can be used in "Sectionalization" of the DS1 path. Sectionalization is a process by which several sections (or "legs") of a data path are analyzed to determine in which leg a communication problem originates. In particular, for the purposes of this discussion, sectionalization refers to determining whether a problem in the DS1 path originates within the CPE equipment 40 (for which the customer is responsible), or in the network (i.e., the LEC equipment 42, 44, for which the local exchange carrier is responsible, or the T1 network equipment 52, for which the Interexchange Carrier (IEC) is responsible). However, since PRMs are transmitted in the DL channel (which only exists for ESF formatted DS1 channels), only equipment that conforms to the ESF format described in ANSI T1.403 can transmit PRMs.
An NIU (referred to as a Remote Module) described in U.S. Pat. No. 5,566,161, which is assigned to Applied Digital Access, Inc. of San Diego, Calif. provides a means by which SF formatted signals received from the CPE can be converted into ESF formatted signals and ESF formatted signals received from the network can be reformatted back into SF signals before transmission to the CPE at the other end of the data path. The Remote Module also provides additional information which can used to aid in sectionalization, as described in U.S. patent application Ser. No. 08/713,027. In addition T3AS test and performance monitor equipment, described in U.S. Pat. No. 5,495,470 entitled "Alarm Correlation System for a TelephoneNetwork" and U.S. Pat. No. 5,500,853 entitled "Relative Synchronization System for a Telephone Network", each assigned to the assignee of the present application and each incorporated herein by this reference analyzes the information provided by PRMs, and additional information that is made available by a Remote Module located at the network interface to sectionalize problems non-intrusively, in real-time, without the need to dispatch a technician regardless of whether the CPF is using ESF format.
However, under the following conditions in which a NIU capable of storing information regarding the performance of the circuit is present at one network interface, such non-intrusive, real-time sectionalization is not possible: (1) the circuit has a T3AS or other such equipment capable of interpreting the information that can be provided by a Remote Module or other NIU, but at least one network interface lacks an NIU capable of performing frame format conversion; (2) the circuit does not have a T3AS or other equipment which is capable of utilizing the information that is provided within the PRMs and/or which is output by the Remote Modules at the network interface at each end of the circuit; and (3) both a T3AS and at least one NIU capable of performing frame format conversion are lacking.
Under each of these three conditions, in order to determine where trouble on the circuit has originated, information must be retrieved from the network interface. Under the first of these three conditions, if one NIU capable of providing information regarding the performance of the circuit over an ESF DL is present, and the circuit is using ESF format, then information can be retrieved with regard to that portion of the circuit between the T3AS and the NIU over the ESF DL, as described in U.S. Pat. No.5,495,470 entitled "Alarm Correlation System for a Telephone Network" and U.S. Pat. No. 5,500,853 entitled "Relative Synchronization System for a Telephone Network", each assigned to the assignee of the present application and each being incorporated herein by this reference. However, if the circuit is not using ESF format, then prior art intrusive methods for retrieving the information stored within the NIU must be used.
Under either the second or third of these three conditions, even through information regarding the performance of the circuit at the network interface may be available from each NIU to equipment specially designed to receive such information, if no such specially designed equipment is present to decode the information, the information cannot be utilized without using prior art intrusive methods for retrieving the information from each NIU.
Referring again to FIG. 2, there are several types of NIUs 50 currently in use which store information for later retrieval. One of the most popular types of NIUs 50 is the "Smart Jack" available from Westell, Inc., located in Oakbrook, Ill. The Smart Jack NIUJ with Performance Monitoring (PM) allows the ILECs to determine what errors are received and generated by the CPE 40. The Smart Jack NIU accumulates PM data and stores the data in a local buffer for later retrieval by LEC personnel. However, data retrieval in most areas requires that a circuit be taken completely out-of-service and that the NIU be commanded intrusively using a proprietary command set. Furthermore, the Smart Jack NIU disadvantageously provides no practical method for the LECs to retrieve the performance monitor data collected by the Smart Jack NIU. While the Smart Jack NIU does allow non-intrusive transmission of PM data from the Smart Jack to the central office, a paralleling maintenance line must be provided. Most DS1 installations to customer premises, however, do not provide such maintenance lines.
Other NIUs 50 are available from Wescom Integrated Network Systems (WINS), the Larus Corporation, and Teltrend, Inc. All of the prior art NIUs 50 suffer the disadvantages associated with out-of-service monitoring and testing. Therefore, there is a need for an improved NIU 50 which provides non-intrusive maintenance performance monitoring at the point of demarcation between the CPE 40 and the LEC equipment.
In addition to being unable to provide non-intrusive retrieval of information collected during real-time monitoring of DS1 digital equipment, the prior art NIUs 50 a desirable indication of a loss of signal (LOS) caused by the CPE 40 which is distinguishable from LOSs that are caused by failure of the network equipment. Currently, LOS caused by the CPE 40 generates alarms in the LEC central office equipment 44 which are indistinguishable from the alarms generated in response to LOSs caused by equipment failures in the local loop 42, Central Office 44, or DS1 network 52. Therefore, there is a need for an improved NIU which allows the LECs to distinguish LOS alarm signals caused by loss of signal within the CPE 40 from alarms which originate due to loss of signal within the LEC or network. With such an improved NIU, the LECs can then decide whether to notify their customers of the LOS indication or to ignore the indication as they deem appropriate.
Furthermore, unless there is a reason to suspect a problem on a particular data path, the data is not requested. In addition, in prior art data paths in which data is collected and transmitted at regular intervals (such as by PRMs generated by a CSU) this data is not collected at the network interface and therefore the ability to sectionalize the data path does not correlate with the portions of the data path which different organizations are responsible for maintaining.
Therefore there is a need for an improved NIU which allows telephone companies to monitor the performance of a DS1 circuit at the network interface and to recover the performance monitoring data non-intrusively even on circuits that are limited to using SF frame format.
Accordingly, it would be desirable to provide a system which sectionalizes a DS1 path to allow each of the parties responsible for maintaining equipment along the path to determine which party is responsible for the problem. Furthermore it would be desirable to reduce the cost of sectionalizing a DS1 path by determining the location of the origin of the problem without sending a highly skilled technician to a plurality of locations along the path.